A conventional low pressure casting machine comprises a holding furnace having a holding chamber substantially filled with molten metal and a vented mold or other molten metal-receiving member mounted on top of a pressure-tight furnace lid or cover. The mold or other molten metal-receiving member is mounted on a fixture that is in communication with a riser tube that extends through the furnace lid and into the molten bath. A gas under pressure is introduced into the holding furnace chamber above the molten metal bath whereupon the molten metal flows upwardly through the riser tube into the mold. Such machines are called "low pressure" casting machines because the pressure exerted on top of the metal bath within the holding furnace is only on the order of three to ten pounds per square inch above atmosphere.
Low pressure casting processes are essentially non-turbulent. Since molten aluminum which has been agitated, particularly in air, is less dense and of lower quality because of higher levels of oxide inclusions than metal which has not been agitated, parts produced by low pressure casting processes are often denser and of higher quality than parts produced by other casting operations.
Although there is minimal agitation of the aluminum during a low pressure casting operation, there is a problem encountered with known low pressure aluminum casting operations because there is no satisfactory way to deliver molten metal to the low pressure holding furnace which does not cause the molten metal to be exposed to air and agitated during the delivery process. To fill a low pressure holding furnace with molten metal, molten metal which has been transferred out of a metal melting furnace (or a holding furnace located adjacent the low pressure casting machine), is poured into the low pressure holding furnace by a transfer device, such as a ladle. To do this, it is usually first necessary to open a pressure-tight cover over the holding furnace, transfer the molten metal into the holding furnace, and replace the pressure-tight cover. During these operations, the metal is agitated by the transferring and pouring operations so that molten metal in the holding furnace may already be significantly agitated before the casting operations are begun. These are time-consuming and expensive operations which may produce parts having less than the desired quality.
In a typical aluminum die casting system, aluminum melted by a melting furnace is delivered to a holding furnace adjacent a die casting machine so that the metal used for die casting is ladled out of holding furnace. Improved quality of die-cast metal parts is obtained by delivering the molten metal to the holding furnace (or holding furnaces) by a launder assembly which relies on the property of liquids to maintain a uniform level. If an installation has one or more aluminum melting furnaces and one or more holding furnaces connected by a launder assembly, the level of the molten aluminum throughout the system remains substantially constant. When molten aluminum is removed from a holding furnace for a die casting operation, the level of the molten aluminum temporarily lowers but immediately begins to return to its normal level as the melting furnace replenishes the aluminum in the system. Some launder assemblies have fairly short launder troughs which need not be heated to maintain the aluminum in a molten state. In other cases, the launder troughs are so long that they are provided with spaced heating elements for maintaining the aluminum in a molten condition.